This invention relates to a synthetic quartz glass member for excimer lasers, and more particularly, to a synthetic quartz glass member having a minimal change of light transmittance and useful as optical parts such as lenses, prisms, mirrors, and windows in excimer lasers, especially ArF excimer lasers, and substrate materials in photomasks. It also relates to a method of preparing such a synthetic quartz glass member.
To meet the recent trend of LSI toward higher integration, the photolithography of defining an integrated circuit pattern on a wafer requires an image exposure technique on the order of submicron units. For finer line width imaging, efforts have been made to reduce the wavelength of a light source of the exposure system. Then, the stepper lens for lithography, for example, is required to have excellent homogeneity and UV transmittance, and high resistance to UV irradiation.
From such a standpoint, synthetic quartz glass having a minimal content of impurities is used. In order to avoid contamination with metal impurities which cause UV absorption, the synthesis of quartz glass is generally carried out by introducing the vapor of a high purity silicon compound such as silicon tetrachloride directly into an oxyhydrogen flame. Flame hydrolysis takes place to form silica fines, which are directly deposited on a rotating heat-resistant substrate such as quartz glass and fusion vitrified thereon. In this way, transparent synthetic quartz glass is produced.
The transparent synthetic quartz glass thus produced exhibits satisfactory light transmittance in the short-wavelength range down to about 190 nm. It has been utilized as materials capable of transmitting UV laser light, specifically i-line and excimer laser light such as KrF (248 nm), XeCl (308 nm), XeBr (282 nm), XeF (351 and 353 nm) and ArF (193 nm), and the four-fold high harmonic wave (250 nm) of YAG.
The absorption of light in the UV region newly created by irradiating synthetic quartz glass with UV light having great energy as emitted by an excimer laser is deemed to be due to the paramagnetic defects formed through optical reaction from intrinsic defects in synthetic quartz glass. Many light absorption bands due to such paramagnetic defects have been identified by ESR spectroscopy, for example, as Exe2x80x2 center (Sixe2x80xa2) and NBOHC (Sixe2x80x94Oxe2x80xa2).
The paramagnetic defects generally have an optical absorption band. When UV light is irradiated to quartz glass, the problematic absorption bands in the UV region due to paramagnetic defects in quartz glass are, for example, at 215 nm due to Exe2x80x2 center (Sixe2x80xa2) and 260 nm, which has not been accurately identified. These absorption bands are relatively broad and sometimes entail outstanding absorption. This is a serious problem when quartz glass is used as a transmitting material for ArF and KrF excimer lasers.
Intrinsic defects in synthetic quartz glass which cause paramagnetic defects arise from structures other than SiO2 such as Sixe2x80x94OH and Sixe2x80x94Cl and oxygen-depleted or enriched structures such as Sixe2x80x94Si and Sixe2x80x94Oxe2x80x94Oxe2x80x94Si.
As the approach for suppressing paramagnetic defects, it is proposed in JP-A 6-199532 to use a chlorine-free alkoxysilane such as tetramethoxysilane as the silane compound for preventing Sixe2x80x94Cl, one of paramagnetic defects, from being incorporated in glass.
It is also effective to utilize the phenomenon that synthetic quartz glass, when irradiated by an ArF laser, experiences an abrupt drop of transmittance at about 1xc3x97104 shots in an initial stage, but as irradiation continues, recovers the transmittance at about 1xc3x97106 shots. This phenomenon arise from a hydrogen concentration in the glass. The drop of transmittance at the initial stage of laser irradiation is greater with a higher hydrogen molecule concentration, but less with a lower hydrogen molecule concentration. The phenomenon is inverse in the case of long-term irradiation, that is, the drop of transmittance during long-term laser irradiation is less with a higher hydrogen molecule concentration, but greater with a lower hydrogen molecule concentration. This is presumed to be in accord with the following formula (4) or (5) as described in JP-A 7-43891.
Initial stage of irradiation
Sixe2x80x94H+hxcexdxe2x86x92Sixe2x80xa2+Hxe2x80x83xe2x80x83(4)
Recovery
Sixe2x80xa2+Hxe2x86x92Sixe2x80x94Hxe2x80x83xe2x80x83(5)
It is also proposed in JP-A 6-305736 to control the hydrogen molecule concentration in synthetic quartz glass. Depending on the energy using conditions of an ArF laser, the hydrogen molecular concentration in glass is adjusted. However, the adjustment of only the hydrogen molecule concentration is still not sufficient to suppress a change of transmittance, leaving a practical problem.
An object of the invention is to provide a synthetic quartz glass member for excimer lasers, having minimized the change of light transmittance in the UV region caused by irradiation of intense UV, and a method for preparing the same.
It has been found that when a synthetic quartz glass having a change of transmittance at wavelength 193 nm of up to 0.002 cmxe2x88x921 as expressed in extinction coefficient when 4xc3x97104 shots of ArF excimer laser light are irradiated at 2 mJ/cm2/pulse, an initial transmittance of at least 99.6% at 193 nm, a hydrogen molecule content of at least 5xc3x971017 molecules/cm3, a refractive index amplitude of up to 1xc3x9710xe2x88x926, and a birefringence of up to 1 nm/cm is used as a synthetic quartz glass member for an excimer laser, especially as a synthetic quartz glass member for an ArF excimer laser or a synthetic quartz glass member for a photomask, the change of light transmittance in the UV region caused by irradiation of intense UV is restrained.
In a first aspect, the invention provides a synthetic quartz glass member for an excimer laser, having (i) a change of transmittance at wavelength 193 nm of up to 0.002 cmxe2x88x921 as expressed in extinction coefficient when 4xc3x97104 shots of ArF excimer laser light are irradiated at 2 mJ/cm2/pulse, (ii) an initial transmittance of at least 99.6% at 193 nm, (iii) a hydrogen molecule content of at least 5xc3x971017 molecules/cm3, (iv) a refractive index amplitude of up to 1xc3x9710xe2x88x926, and (v) a birefringence of up to 1 nm/cm.
Typically the synthetic quartz glass member is for use in an ArF excimer laser. Also it is preferably used as a substrate for photomask.
In a second aspect, the invention provides a method for preparing the synthetic quartz glass member, comprising the step of feeding a mixture of an organic silicon compound and oxygen into a central nozzle of a burner for effecting decomposition of the organic silicon compound with an oxyhydrogen flame to synthesize quartz glass, the mixing ratio of the organic silicon compound to oxygen being at least two fold moles of the theoretical amount of oxygen. In another embodiment, the method further involves the step of annealing the resulting synthetic quartz glass member. In a further embodiment, the method further involves the steps of homogenizing and then annealing the resulting synthetic quartz glass member.